MEMS or Micro Electro Mechanical Systems have become useful in a variety of fields. These MEMS have been applied to such technologies as inkjet printers, accelerometers, microphones, optical and electrical switching, and fluid acceleration. Over the last decade, there has been a focus towards the development of a subclass of these devices, termed Micro-Opto-Electro-Mechanical Systems (MOEMS).
One type of MOEMS device is an electrostatically deflectable membrane. Such MOEMS membranes are used in a variety of optical applications. For example, they can be coated to be reflective and then paired with a stationary mirror to form a tunable Fabry-Perot (FP) cavity/filter. They can also be used as stand-alone reflective components to define the end of a laser cavity, for example.
Typically, a voltage is applied between the membrane and an adjacent structure. When paired with a second fixed reflector, the FP cavity's separation distance changes through electrostatic attraction as a function of the applied voltage.
There are a few main components that typically makeup a MOEMS membrane device. In one example, the MOEMS membrane device includes a handle wafer support structure. An optical membrane layer is added to the handle wafer support structure; a deflectable membrane structure is then fabricated in this layer. This MOEMS membrane device includes an insulating layer separating the wafer support structure from the membrane layer. This insulating layer is partially etched away or otherwise removed to produce the suspended membrane structure in a release process. The insulating layer thickness defines an electrical cavity across which electrical fields are established that are used to electrostatically deflect the membrane structure.
One major problem with the many MOEMS membrane devices is “pull-in” instability. Pull-in voltage is understood as the voltage that results in an electrostatic force that causes a membrane structure to be pulled against a nearby surface. The instability arises when a membrane structure moves inward and the electrostatic forces overtake the mechanical restoring forces of the membrane structure. This can cause the membrane structure to snap-down uncontrollably into an adjacent surface such as the wafer support structure and sometimes even adhere to it through a process of stiction adhesion. Stiction is a strong attraction force that causes the adhesion of two elements to one another to the point of being almost unbreakable and results from Van der Waals forces, among others.
This problem can be especially intractable in the context of optical membrane structures of MOEMS devices. This is because anti-stiction coatings are typically incompatible with the required optical coatings, such as antireflective (AR) coatings or dielectric highly reflecting (HR) coatings, for example. Moreover, MOEMS membrane structures are typically especially smooth to maximize optical performance. The smoothness of the membrane typically increases the level of stiction forces in the event of contact.
There have been MOEMS membrane device designs that have tried to combat stiction adhesion. In one example, a MOEMS membrane device includes stiction plugs formed into the membrane structure and arranged so that the plugs project towards the adjacent support structure. Therefore, if the membrane comes in contact with the adjacent support structure, the stiction plugs first contact the adjacent surfaces preventing stiction adhesion of the membrane structure to the support structure.